1. Field of the Invention
The present invention relates to a device for receiving an orthogonal frequency division multiplexing (OFDM) signal, and a method for restoring the signal by channel estimation, and in particular to an improved device for receiving an OFDM signal which can restore the signal by estimating a property of a channel where the signal is transmitted, and a method for restoring the signal by channel estimation.
2. Description of the Related Art
Orthogonal frequency division multiplexing (OFDM) modulation converts serial data into parallel data, performs fast Fourier transform (FFT) thereon, converts the transformed data into serial data, and performs this conversion in an inverse order.
FIG. 1 is a block diagram illustrating a conventional device for receiving an OFDM signal.
Referring to FIG. 1, in order to restore the consecutively-inputted OFDM signals, the device for receiving the OFDM signal includes an ADC 10 for A/D conversion; a detection unit 11 for detecting a start sample of an OFDM symbol; an FFT unit 12 for performing the FFT; a delay unit 13 for delaying the received symbol; first and second storing units 15, 16 for respectively sequentially storing output symbols from the FFT unit 12; a channel estimation unit 17 for estimating a channel; and an equalizing unit 14 for compensating for distortion of the reception signal.
In the conventional device for receiving the OFDM signal, the ADC 10 converts an inputted analog signal into a digital signal, and outputs the digital signal into the detection unit 11.
The detection unit 11 detects the start sample of the OFDM symbol in the sample column from the ADC 10, and outputs the start sample to the FFT unit 12.
The FFT unit 12 performs the FFT on the samples from the detection unit 11, and outputs the transformed samples to the first and second storing units 15, 16.
The first and second storing units 15, 16 store the signals from the FFT unit 12.
Here, the first storing unit 15 stores the signals from the second storing unit 16. Accordingly, the m-th symbol is stored in the first storing unit 15, and the m+1th symbol is stored in the second storing unit 16.
The channel estimation unit 17 estimates a property of the channel by calculating an average of the m-th symbol and the m+1th symbol in the first and second storing units 15, 16, and transmits the property of the channel to the equalizing unit 14. According to the estimated property of the channel, the equalizing unit 14 compensates for distortion of the m-th signal delayed by the delay unit 13.
Here, “channel” implies an information transmission path between devices. In addition to physical channels, logical channels may be formed.
In general, the property of the transmission channel is obtained by using a transmission signal and a reception signal, which is represented by following formula 1.
                              H          ⁡                      (                          k              ,              m                        )                          =                              Y            ⁡                          (                              k                ,                m                            )                                            X            ⁡                          (                              k                ,                m                            )                                                          <                  Formula          ⁢                                          ⁢          1                >            
Here, H(k,m) is a function representing a channel property for the k-th subcarrier frequency of the m-th OFDM symbol, X(k, m) is a parameter showing a property of the transmission signal for the k-th subcarrier of the m-th OFDM symbol, and Y(k,m) is a parameter showing a property of the reception signal for the k-th subearrier of the m-th OFDM symbol.
In addition, the channel property (H(k,m)) includes a phase component, as in following formula 2.H(k,m)=|H(k,m)|·ejΦH(k,m)  <Formula 2>
As shown in formula 2, the channel property is dependent upon the subcarrier frequency (K) and the transmission time (m). For example, the magnitude of the transmission signal is varied by |H(k,m)| times, and the phase thereof is rotated by ΦH(k,m).
On the other hand, the respective channel properties for the m-th and m+1th symbols are obtained according to the generally-known channel estimation method, using the transmission and reception signals of the subcarrier as shown in formula 2. Thereafter, the channel property of the m-th signal can be estimated by using an average thereof, as shown in following formula 3.
                              H          ⁡                      (            k            )                          =                              1            2                    ⁢                      {                                          H                ⁡                                  (                                      k                    ,                    m                                    )                                            +                              H                ⁡                                  (                                      k                    ,                                          m                      +                      1                                                        )                                                      }                                              <                  Formula          ⁢                                          ⁢          3                >            
However, the sampling time of the receiving device is varied in every sampling period due to a sampling clock offset generated in sampling of the reception signal. Accordingly, an interference occurs between the subcarriers, and thus the phase variations for the sampling time are increased in proportion to the subcarrier frequency. In consideration of these phenomena, the phase value is represented by following formula 4.
                              Φ          ⁡                      (                          k              ,              m                        )                          =                              2            ⁢            Π            ⁢                                                  ⁢            k            ×                                          τ                m                            NT                                +                                    Φ              p                        ⁡                          (              m              )                                +                                    Φ              H                        ⁡                          (              k              )                                                          <                  Formula          ⁢                                          ⁢          4                >            
Here, Φ(k,m) is a phase generated in the k-th subcarrier of the m-th symbol, which is a phase ΦH(k) distorted due to a sampling clock offset τm, a phase noise Φp(m) and a transmission channel, as shown in FIG. 2A.
On the other hand, a phase generated in the k-th subcarrier of the m+1th symbol is represented by following formula 5, using formula 4.
                                                                        Φ                ⁡                                  (                                      k                    ,                                          m                      +                      1                                                        )                                            =                                                2                  ⁢                  π                  ⁢                                                                          ⁢                  k                  ×                                                            τ                                              m                        +                        1                                                              NT                                                  +                                                      Φ                    p                                    ⁡                                      (                                          m                      +                      1                                        )                                                  +                                                      Φ                    H                                    ⁡                                      (                    k                    )                                                                                                                          =                                                2                  ⁢                  π                  ⁢                                                                          ⁢                  k                  ×                                                                                    τ                        m                                            +                                              Δ                        ⁢                                                                                                  ⁢                        τ                                                              NT                                                  +                                                      Φ                    p                                    ⁡                                      (                    m                    )                                                  +                                                      ΔΦ                    p                                    ⁡                                      (                                          m                      +                      1                                        )                                                  +                                                      Φ                                          H                      )                                                        ⁡                                      (                    k                    )                                                                                                          <                  Formula          ⁢                                          ⁢          5                >            
It is recognized that formula 5 shown in FIG. 2B has a different sampling clock offset and phase noise from formula 4.
Accordingly, as shown in formula 3, the conventional method for estimating the channel property of the reception signal without considering the offset amount of the sampling clock timing and the common phase noise cannot precisely estimate the property of the transmission channel for restoring the transmission signal.
That is, as illustrated in FIG. 2C, the phase distortion for each subcarrier of the m-th and m+1th OFDM symbols has a different phase from the transmission channel distortion due to the offset amount of the sampling clock timing and the common phase noise. As a result, the conventional method for estimating the property of the OFDM channel cannot precisely restore the original signal.